Scientific Basis And Implications

In addition to its role in the development of HACCP plans, hazard characterization has been identified as the second step of the risk assessment process (Smith et al., 1999). The characterization includes determination of risk factors, defining the site and mechanism of action, and measuring the dose-response relationship (proportion responding or severity of response). Despite large uncertainties, dose-response models are commonly used to predict human health effects and even to establish regulatory policies.

According to the WHO (1995), a dose-response assessment should be performed for chemical hazards. For biological or physical agents, a dose-response assessment should be performed if the data are obtainable. Although potentially hazardous chemicals may be present in foods at low levels, for example, parts per million or less, animal toxicological studies typically are done at higher levels to obtain a measurable effect. The significance of the adverse effects associated with high-dose animal studies for low-dose human exposure is a major topic of debate with regard to the hazard characterization of chemicals.

The extrapolation of animal exposure data to human exposure levels is uncertain both qualitatively and quantitatively. The nature of the hazard may change with dose. Not only is the equivalent dose estimate in animals and humans problematic in comparative pharmacokinetics, the metabolism of the chemical may change as the dose changes. Whereas high doses can overwhelm detoxification pathways, the effects may be unrelated to those seen at low doses (WHO, 1995).

A primary contributor to the uncertainty of the hazard characterization is the intraspecies variance in the dose response at different dosage levels. Large exposures often are used to increase the power of a study yet may be inaccurate for low-dose exposure. Variance also results from many other differences among individual animals and humans.

Toxicologists often use thresholds to quantify adverse effects from chemical exposures, except in the case of carcinogenic effects, where initiating events can occur as persistent somatic mutations that later develop into cancer. Some carcinogens may be regulated with a threshold approach, such as the "No Observed Effect Level (NOEL)-safety factor" approach. A safe level of a chemical often is derived from an experimental NOEL or No Observed Adverse Effect Level (NOAEL) by using safety factors. A safety factor of 100 has been applied when using data from long-term animal studies, but it may be adjusted if data are insufficient or if the effect is more severe or irreversible. It has been suggested that conservative models and large safety factors should be used for food systems potentially contaminated with biological hazards because of the unpredictability of these systems (Smith et al., 1999). Obviously, the safety factor approach is full of uncertainty and cannot guarantee absolute safety for everyone.

For carcinogens that cause genetic alterations in target cells, the NOEL safety factor approach is usually not used because of the assumption that risk exists at all doses, even the lowest. Risk management options are to ban the chemical or establish a negligible, insignificant, or socially acceptable level of risk with quantitative risk assessment. An alternative approach has been to use a lower effective dose, or a benchmark dose, which depends more on data near the observed dose-response range. This may allow more accurate predictions of low-dose risks.

Characterization of biological hazards is done to provide a qualitative or quantitative estimate of the severity and duration of adverse effects due to the presence of a pathogen in a food. Dose-response data are useful but scarce for microbial pathogens. Furthermore, inaccuracies in the data may occur for the following reasons: host susceptibility to pathogenic bacteria is variable; attack rates from specific pathogens vary; virulence of a pathogen is variable; pathogenicity is subject to genetic mutation; antagonism from other microbes may affect pathogenicity; and foods will modulate microbial-host interactions.

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